Photo-catalytic reactor

ABSTRACT

A photocatalytic reactor, capable of generating an electric current by consumption of a fuel containing organic material, comprises a direct oxidation fuel cell including an anode and a cathode. The anode is a photocatalysis-assisted anode which comprises a photocatalyst on a surface of an electrically-conductive substrate so arranged as to be receptive to light. A light-transmissive proton-conductive membrane is arranged between the anode and the cathode, such that light passes through said membrane as a final stage in the optical path to the photocatalyst. The photocatalyst promotes oxidation of organic material and generates electron-hole pairs. The reactor, configured to support multiple cells in a stacked array, is provided with inlet(s) for introducing said fuel and connector(s) for connection to an external electrical circuit.

FIELD OF THE INVENTION

The present invention generally relates to use of fuel cells, and inparticular liquid feed organic fuel cells wherein oxidation of the fuelis achieved by photocatalysis. The invention to be particularlydescribed hereinafter provides an energy efficient photocatalyticreactor.

BACKGROUND OF THE INVENTION

Fuel cells are electrochemical cells in which a free energy changeresulting from a fuel oxidation reaction is converted into electricalenergy. In an organic/air fuel cell an organic material such as methanolor other suitable fuel is oxidised to carbon dioxide at the anode whilstoxygen from air, or oxygen enriched air, or oxygen gas itself, isreduced to water at the cathode.

Two types of organic/air fuel cells are generally known:

-   -   1. An indirect fuel cell in which the organic fuel is        catalytically reformed and processed into hydrogen, which is        used as the actual fuel for the fuel cell by being oxidised at        the anode.    -   2. A direct oxidation fuel cell in which the organic fuel is        directly fed into the fuel cell and oxidised at the anode which        typically employs platinum group metals or alloys containing        platinum group metal as the catalyst.

Direct oxidation fuel cells are currently the subject of substantialinterest for use in a wide variety of applications. Such cells have thepotential for providing useful energy outputs in a “clean” and efficientmanner using renewable fuels such as methanol. Such fuels can beobtained, for example, by biomass fermentation processes.

Difficulties encountered in producing a practical direct fuel cellinclude:

-   -   Catalyst and electrode design and efficiency, avoiding poisoning        and minimising the production of undesirable side products such        as carbon monoxide;    -   Efficiency of the cathode, especially if air is used as the        oxygen containing gas, the nitrogen present can ‘blanket’ or        slow down the transport of the oxygen to the catalyst surface;    -   Fuel ‘cross-over’, i.e. the anode and cathode of the cell are        separated by an tonically conductive medium such as a high        molecular weight electrolyte or solid proton conducting        membrane, but if the fuel can permeate that membrane and be        transported from the anode to the cathode then efficiency is        lost; and    -   Choice of electrolyte—direct oxidation fuel cells often employ        sulphuric acid as the electrolyte but the consequent presence of        sulphate ions and sulphur can result in poor performance.

In U.S. Pat. No. 5,599,638 an improved type of cell using a solidpolymer electrolyte and improved electrodes is described, with yetfurther improvements being revealed in U.S. Pat. No. 6,303,244 by thesame inventors. A notable feature of that work is the use of a solidpolymer electrolyte, a perfluorosulphonic acid containing polymer, suchas “Nafion®”. This avoids the use of sulphuric acid electrolyte andgives improved performance from the cell.

In U.S. Pat. No. 5,094,927 an alternative solid electrolyte isdescribed, a proton conducting solid comprising at least one oxide of anelement selected from Group IVB, VB, VEB, and VIII elements of thePeriodic Table, silicon dioxide, and at least one fluoride of an elementselected from the elements in Group IIA and IIIB of the Periodic Table.Such an electrolyte is proposed as a feature of an indirect(hydrogen/oxygen) fuel cell in that patent.

A disadvantage of the known types of fuel cell is that they generallyrequire highly purified fuel to prevent catalyst poisoning. The fuelcell of U.S. Pat. No. 6,303,244 requires highly pure methanol, theinventors envisage having to fit filtration systems to removehydrocarbon traces from the fuel when their invention is used in anautomotive environment.

An object of the present invention is to provide improvements in orrelating to fuel cells, whereby the aforesaid disadvantages of the priorart are obviated or mitigated.

A further object of the invention is to provide a direct oxidation typeof liquid feed fuel cell that utilises photo-catalysed oxidation at theanode.

Another object of the invention is to provide means of gathering anddirecting light to the photocatalytic anode.

A yet further object of the invention is to provide a photocatalyticreactor that can be utilised in the intended destruction of organiccompounds present in waste streams from industrial processes in anenergy efficient manner.

SUMMARY OF THE INVENTION

Accordingly, the aforesaid objects are addressed in that the presentinvention provides a photocatalytic reactor which includes features of amodified direct oxidation fuel cell wherein the oxidation of the fuel iscarried out at an anode that comprises a photocatalyst on a conductingsubstrate. It will be recognised that by this application of thephotocatalyst, one obtains an anode where the photocatalyst is used toinduce the necessary charge separation to allow reaction with the fuel,i.e. the function of the photocatalyst is to produce electrons andholes. Those skilled in the art will recognise that the electrons areremoved by the external circuit and the holes produce protons byinteraction with the fuel. This approach to the oxidation step offersadvantages in that a different catalyst technology is employed incomparison with a conventional direct oxidation fuel cell with theability to use a wide range of fuels, even contaminated fuels, inprospect.

Thus according to the present invention there is provided aphotocatalytic reactor capable of generating an electric current byconsumption of a fuel containing organic material, said reactorcomprising a direct oxidation fuel cell including an anode and acathode, wherein the anode is a photocatalysis-assisted anode whichcomprises a photocatalyst on a surface of an electrically-conductivesubstrate so arranged as to be receptive to light, and aproton-conductive membrane arranged between said anode and the cathode,such that light passes through said membrane as a final stage in anoptical path to the photocatalyst, the said photocatalyst being capableof promoting the oxidation of organic material and generatingelectron-hole pairs, and said reactor is provided with means forintroducing said fuel, and means for connection to an externalelectrical circuit.

The cathode maybe selected from a mesh, a porous element or a perforatedstrip, and the material thereof is a noble metal, e.g. platinum orsilver, or catalytic metals or alloys known in the art as suitable forthis purpose, or more modern materials such as ceramics.

The reactor is preferably configured to support multiple fuel cells ofthe aforesaid type in a stacked array.

The arrangement of the photocatalyst to receive light may involve anoptical path wherein the aforesaid proton-conductive membrane isjuxtaposed with further light-conductive materials e.g. so-called “lightpipes” to enhance the delivery of light to the photocatalytic surface.Such an arrangement facilitates the presentation of a plurality ofphotocatalytic cells in a battery or stack preferably of thin (0.3-0.5mm) cells. Where light pipes are used, cell thickness may increase toapproximately 1 mm or more. The light source is preferably natural light(solar energy), but artificial light sources may be also provided. Theoperation of the invention may be improved by provision of lightgathering and intensification optics.

Examples of suitable materials that can serve as photocatalysts for thepurposes of this invention include but are not limited to titaniumoxides, titanium oxides doped with nitrogen, tungsten oxides, mixedoxide systems such as titanium oxides in combination with tungstenoxides or molybdenum oxides, or indium nickel tantalates. It ispreferred that the photocatalyst comprises elements exhibiting stablemixed oxidation states.

It is observed for the purposes of better understanding of the inventionthat although in this application and in the literature, reference ismade to these materials as “photocatalysts”, the strict position is thatthese materials operate as light-assisted oxidising agents which onlyexhibit catalytic properties in the electrochemical cell. Given thatoxidation of fuel results in reduction of the “photocatalyst” metal(tungsten, titanium, molybdenum, vanadium, etc), M^(z) to M^(z-1), it isbelieved that the cathode reaction re-oxidises the metal by withdrawingthe extra electrons (generated by fuel oxidation) such thatM^(z-1)→M^(z)+e. Without this cathode effect, the concentration of thereduced form of the metal would increase to some saturation level (whichdepends on the relative stabilities of M^(z) and M^(z-1) (e.g.Ti⁴⁺/Ti³⁺, W⁶⁺/W⁵⁺), resulting in a change in oxide stoichiometry overtime, i.e. not a true catalyst in the purest sense of the term.Consequently, one need not look amongst the limited class of truecatalysts to identify material that would provide suitablephotocatalytic effects for implementation of the invention describedherein.

Although a photocatalytic fuel cell is already described in JapanesePatent 59165379, it is considered that this does not offer advantages ofthe fuel cell to be more particularly described hereinafter. That patentdescribes a fuel cell that uses organic substances, such as sodiumformate solution, as fuel. The anode consists of a cadmium sulphide(CdS) single crystal that acts as a reactive oxidative surface whenirradiated with ultra-violet light, shone into the cell via a quartzwindow. The cell is completed with a platinum black cathode immersed ina sulphuric acid electrolyte and an agar salt bridge to connect theanode and cathode chambers. Disadvantages of such an arrangement includethe poor efficiency of the CdS photocatalytic surface, the limited scopeof organic compounds that can be used and the need for quartz windowedchambers and ultra-violet light. Although more recent publicationsdescribe similar but more efficient photoelectrocatalytic cells, therequirement for ultra-violet light sources and quartz apparatuspersists.

In contrast the present invention makes use of a range of photocatalystssuch as titanium oxides that have been developed for use in thephotocatalytic destruction of organic compounds. Such metal oxidematerials can be modified to interact with visible rather thanultra-violet light, for example by nitrogen-doping or the introductionof other species such as other metal oxides into the catalystcomposition.

Improved means of supplying the light to the photocatalytic surface arealso provided by the present invention by use of light guides orconduits (“light pipes”) as described hereinafter.

According to a further aspect of the invention there is provided amethod of generating electrical power, particularly by consumption of anorganic fuel, by a photocatalytic reaction conducted in a directoxidation fuel cell, said method comprising the provision of a fuel celland a source of fuel for the cell, wherein the anode of the cell is aphotocatalysis-assisted anode which comprises a photocatalyst on asurface of an electrically-conductive substrate so arranged as to bereceptive to light, and a light-transmissive, proton-conductive membranearranged between said anode and the cathode, such that light passesthrough said membrane as a final stage in an optical path to thephotocatalyst, the said photocatalyst being capable of promoting theoxidation of organic material and generating electron-hole pairs,exposing the photocatalytic surface to light, and supplying fuel to theanode for photocatalytic oxidation, and generating electrical power as aresult of the said photocatalytic oxidation of the fuel.

It should be understood that the “fuel” that can be used for thepurposes of the invention is not limited to methanol or indeed otheralcohols but can include use of other organic substances in a fluid formto permit pumping and delivery thereof via conduits. The “cell”described herein has already demonstrated degradation of robustenvironmental pollutants, e.g. herbicides, pesticides, pathogens,endocrine disruptors and toxic bi-products arising from degradation oflandfill constituents and contaminated land. Thus the invention isideally suited for use in the water quality industry.

The fuel cell designed according to the principles of this inventionconsists of three principal components, namely an anode, aproton-conducting membrane, and a cathode, and the salient featuresthereof are as follows:

(i) Anode:

-   -   The anode comprises a photocatalyst coated onto a conducting        substrate that is preferably perforated or porous to facilitate        access of protons to the proton conducting membrane.        Photocatalytic activity relies on photons, from an external        light source, generating electron-hole pairs at the catalyst        surface (e⁻ and h⁺). The substrate represents a fast electronic        conductor. Its positioning has to be carefully considered with        regard to the purpose of enabling electrons to be removed from        the anode to an external circuit thus inhibiting recombination        with holes and providing external electrical current. The holes        generated interact with fuel (in this example methanol) leading        to its oxidation with consequent production of CO₂ and protons:        CH₃OH+H₂O+6h ⁺→CO₂+6H⁺

(ii) The Proton-Conducting Membrane (PCM):

-   -   The PCM separates the anode and the cathode and is made from a        proton-conducting material, such as are already known in the art        of conventional fuel cells, such as a perfluorosulphonic acid        containing polymer (e.g. Nafion®). However it is preferable to        make use of a proton conducting glass, such as a high        conductivity glass with composition 5% P₂O₅: 95% SiO₂ as        described by Nogami et al in Electrochemistry and Solid State        Letters (2, 415-417, 1999). The advantages of using such a glass        are in reduced potential for crossover of the fuel to the        cathode and most especially the glass can permit the transfer of        light to the anode. The PCM permits proton diffusion by a proton        hopping mechanism and the water content of the porous glass        enhances its conductivity (to typically 10^(−1.5) S·cm⁻¹). The        transmission of light to the anode by using the PCM as a        waveguide can be further improved by appropriate modifications        to the structure of the membrane or supplementary “light pipes”.        For example selectively altering the refractive index        characteristics or constructing the membrane or light pipes of        pieces or fibres of glass with different refractive indices, by        the use of light scattering particles distributed throughout the        membrane, or by other means readily apparent to those skilled in        the manufacture of optical devices, to achieve the optimum flux        of light onto the anode surface. Suitable “light pipes” can be        incorporated in the PCM (or on the surfaces of the anode not        contiguous with the PCM) in order to deliver sufficient light to        the photo-catalytic surface.

(iii) The Cathode:

-   -   The cathode provides the necessary surface for recombination, in        the presence of oxygen from an external source (such as air,        oxygen, oxygen-enriched air, or oxygen enriched fluid), of        electrons from the external circuit and protons transported        across the PCM. The net reaction at the cathode is given by:        O₂+4H++4e−→2H₂O    -   The preferred cathode is, but is not limited to, a fluid        diffusion electrode employing a catalyst such as platinum or        related catalytic metals (such as silver) or alloys such as are        well known in the art or more modern materials such as ceramics.

The fuel cell of the present invention can be constructed in differentshapes to suit the application intended. Application of sufficient lightto the anode can be engineered by the provision of additional means,such as light pipes, or by shaping the component parts or the overallcell assembly appropriately to optimise the ability to direct light(natural or artificial) to the photocatalytic surface of the anode.

In order to produce higher output the fuel cell of the invention can beconstructed as “stacks” i.e. connected in series as will be furtherdescribed hereinafter.

The fuel cell of this invention can be utilised for any of the proposeduses of a conventional direct oxidation fuel cell, provided that asuitable light source is available. For instance the cells, fuelled byan oxidizable substance, preferably a relatively inexpensive organicliquid such as methanol, can be used to power electrically operateddevices. The ability of the photocatalyst types selected for use in thisinvention to oxidise (thereby consuming, degrading, or destroying) awide range of organic materials also allows the invention to be used asan energy efficient method of remediating waste streams, such as aqueouswaste streams, from industrial processes that contain organic materials.The fuel cell can be fed such a waste stream which will be oxidised atthe anode whilst generating electrical energy that can be used to powerrequired equipment such as pumps, or used elsewhere in the event of asurplus being generated. Thereby, two objectives are achievable in thatnoxious materials are disposed of in an environmentally acceptablemanner and, in so doing, useful energy is generated.

Photocatalytic oxidation of hazardous organic pollutants has been agrowing area of environmental technology over the last twenty years. Themineralisation of low molecular weight alcohols and chlorinated alkanes,acetone and the partial oxidation of mycrocystins to less toxic forms inpotable waters has been demonstrated, all using TiO₂-based catalyst.Conventional photocatalytic oxidation processes in this theme generallyutilise photocatalyst slurries. This invention seeks to enhance catalystefficiency by utilising thin films, as in the area ofphotoelectrocatalysis and photovoltaics, connected to an externalelectrical circuit. The prospect of recovering electrical energy fromthe degradative oxidation of organic pollutants is particularlyattractive in terms of sustainability and waste utilisation.

There are, for example, applications of the invention in the oil and gasproducing industry. The technology would be applicable to flow-throughprocesses similar to those currently employed for hydrocycloneseparators and other fluid treatment operations. Upstream organicsinclude aliphatic and aromatic hydrocarbons, “demulsifiers”(urea-formaldehyde, phenolic resins, amines and sulphonates), fattyacids, aldehydes and ketones. The reactor of the present invention isparticularly suited to the destruction of these polluting chemicalswhich are present as small droplets at low residual concentrations whenconventional separating treatments have been applied to the contaminatedaqueous stream.

The invention will now be further described by way of illustration withreference to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIG. 1 shows a schematic drawing of the photocatalytic reactor fuel cellof the invention, in an embodiment utilising methanol as fuel;

FIGS. 2A & B show the construction of three embodiments of the fuelcell, FIG. 2B showing the use of “light pipes” in two differentarrangements to conduct light to the photo-catalytic anode;

FIG. 3 shows a stack or battery of the fuel cells of the invention;

FIG. 4 shows X ray diffraction data for a polytungstic acid materialprepared for use as a photocatalyst in the fuel cell of the invention;

FIG. 5 shows thermo-gravimetric analyses of a polytungstic acid materialprepared for use as a photocatalyst in the fuel cell of the invention;

FIG. 6 shows Fourier Transform Infra Red spectra for a polytungstic acidmaterial prepared for use as a photocatalyst in the fuel cell of theinvention;

FIG. 7 shows a reflectance UV/visible light spectrum of a polytungsticacid material prepared for use as a photocatalyst in the fuel cell ofthe invention; and

FIG. 8 shows photocatalytic activity of a polytungstic acid materialprepared for use as a photocatalyst in the fuel cell of the invention;

FIG. 9 shows transmission electron microscopy (TEM) images andcorrelation between recovered photocurrent and catalyst particle size;and

FIG. 10 shows AC impedance derived data on ‘WO₃-based’ photocatalystmaterials.

MODES FOR CARRYING OUT THE INVENTION

The fuel cell of the present invention will now be described by way ofexample in terms of a cell that uses methanol as the fuel, and performsas a photocatalytic reactor wherein the fuel is oxidised at the anode,releasing protons, but it will be understood that other fuels may beadopted, and that organic contaminants in fluids e.g. oil-polluted watercan serve as “fuel”.

The fuel cell comprises electrodes and proton-conducting membrane unitsas illustrated in FIG. 2. Perforated, porous or grid electrodes ensurethat charge carriers can transit across all interfaces and that allelectrical contacts within the electrochemical cell are continuous sothat one external connection to a mesh or foraminated metallic sheetwill be sufficient (see FIG. 2), but multiple connections are notexcluded. The photocatalyst (1) can be dip-coated or applied by othermeans onto the previously fabricated proton conducting membrane (PCM)(3) having a platinum, or other electronically conducting mesh (2)partially embedded into its surface. The cathode (4) is a mesh or porouselement or perforated strips made from platinum or silver. A protonconducting metal film may separate the PCM from the cathode. Anode (1)and cathode (4) are connected externally via an electrical load.

The light transmittance of the PCM may be supplemented by theincorporation of light pipes (5) (see FIG. 2 Options B). These can be ofsimilar composition to the PCM and are capable of scattering lightdirected via the pipes through the perforated conductor and onto thecatalyst surface. They can also serve to add mechanical robustness tothe assembly which could be typically up to 1 mm thick or more (measuredin the direction of proton transport) but can be of other suitabledimensions depending on the application to which the cell is put. Two ofthe possible arrangements for the light pipes are shown in FIG. 2 optionB. The second arrangement shown, with the light pipes incorporated intothe body of the PCM, has advantages. The mechanical robustness of thestructure of the cell is improved and both electrodes can have a maximumsurface area in contact with the PCM to improve efficiency. Furthermorethe ridged or corrugated shape of the cell further enhances the surfacearea of the cell and hence the potential power output.

Each fuel cell unit as shown in FIG. 2 can be used alone as a singlecell to produce power, but greater effectiveness can be gained by makinga device containing several or many units configured, for example in a“stack” or battery as shown in FIG. 3.

Cell assemblies (1) are mounted back to back such that fuel can beadmitted via feed lines (3) and pressure control valves (2) intophotocatalyst chambers (8). Light is directed into the cells (1)perpendicular to the plane of the diagram by suitable means. Oxidant, asair or oxygen-enriched air (or some other suitable oxidant) is fed viaoxidant lines (5) and pressure control valves (4) into oxidant chambers(9). Electrical current is collected from the conducting meshes orperforated/porous metallic sheets. Fuel and oxidant supplies (6 and 7)can be conditioned to optimise cell performance by the use ofcompressors and heat exchangers (not shown) as appropriate. Note thatmany cells can be mounted into modules capable of connecting anddisconnecting into a flow system. This enables modules to be removed andreplaced, or by-passed to enable servicing, catalyst regeneration orother maintenance. In a preferred embodiment of the present inventionwherein light activation of the catalyst is achieved by illuminationfrom within the PCM great flexibility regarding stack design ispossible. Multiple cells can be mounted in modules such that fuel andoxidant flows can be directed simultaneously onto their respectivesurfaces.

In a preferred embodiment of the invention, considered to offer the bestmode of performance at the present time, the catalyst is a WO₃-basedmaterial the characteristics of which are consistent with a polytungsticacid, which although available commercially as a photochromic material,is further characterised below.

Preparation of a ‘WO3-based’ Catalyst Suitable for use in Performance ofthe Invention Preparation

Ammonium tungstate (0.5 g) (99.999%) (Alfa Cesar) was added to distilledwater (200 mL) with constant stirring at room temperature. The pH wasadjusted to 1 using nitric acid (67% AnalaR®). Precipitation occurredwithin 2 hrs. After this time the stirring was stopped and thewhite/yellow precipitate was allowed to settle for 24 hrs. Most of theliquid was then decanted and the precipitate was dried at 100 degreesfor 2 hrs. The resulting yellow powder was then mixed with 10 ml ofdeionised water and deposited on a gold-coated glass slide area (3cm×2.5 cm); a typical amount of catalyst deposited on a slide was 0.01g. The slide was then heat treated to between 100 and 450 degrees C(normally for 10 minutes) yielding a white-yellow catalyst.

Characterization

X-Ray Diffraction.

FIG. 4 shows the pattern obtained from the precipitated product. Thepeak positions shown are consistent with W0₃ but the broad band fromapproximately 13° to 32°2θ suggests that a less well crystallised (ornano-crystallised) constituent is also present.

Thermogravimetric Data:

FIG. 5 shows the results of thermogravimetric analyses of the catalystmaterial dried at 450° C., subsequently re-dispersed in water and thenre-dried at 100 or 450° C. prior to final equilibration in water.

Although TG-MS data are not shown here, the weight loss in both cases isattributable to water. Since WO₃ does not contain water, it is evidentthat the solid catalyst is not pure WO₃ but is partially hydrated, thetemperature of dehydration suggesting that the water is strongly bound,probably to surfaces, as hydroxide. X-ray data indicate that the levelof well-crystallised hydroxide material is low (below detection limits)but it is suggested that a poorly crystallised surface hydroxide layermay be important in defining the photocatalytic performance of thismaterial. Final drying temp (° C.) Initial weight (mg) Wt. Loss (%) 10036.37 0.33 450 38.86 0.73

It is noted that the water loss associated with the sample heated to450° C. is greater, and this may be related to surface area effects.Transmission electron microscopy reveals a rather broad particle sizedistribution in these specimens limiting the level of certainty that canbe attributed to the interpretation. However, better control of particlesize in products is achievable through modification to the sol-gelpreparative route (see below).

Fourier Transform Infra Red (FTIR)

Catalysts samples were dispersed in a mulling agent (KBr) and pressedinto discs. FTIR spectra were then obtained and are shown in FIG. 6(upper spectrum—sample re-dried at 450° C.; lower spectrum—samplere-dried at 100° C.).

Reflectance UV/Visible Spectroscopy

FIG. 7 shows a reflectance UV/visible light spectrum of the ‘WO₃-based’catalyst, and indicates that the band ‘gap’ corresponds to approximately450 nm. This confirms that the catalyst absorbs radiation in the violetregion of the visible spectrum and is consistent with the observedyellow colour of the catalyst.

Catalyst Performance in the Photocatalytic Fuel Cell

The function of the material as a photocatalyst is measured by itsability to degrade the colour of methylene blue solutions introduced tothe photocatalytic fuel cell. FIG. 8 shows the degradation activity ofthe catalyst (pre-dried 450° C.) as a function of time, with or withoutillumination by visible light. Note that the gradient of the graph isdependent on whether or not the light source is on; steeper slopes areobserved when the catalyst is illuminated. The active catalyst area wasapproximately 6 cm², and the methylene blue solution volume wasapproximately 70 mils. The recovered photocurrent was 18 μA (maximumslope).

The rate of colour degradation in methylene blue was initiallycorrelated with the recovered photocurrent from the cell but it wassubsequently shown, through adaptation (Santato, C., et al, J. Am. Chem.Soc., 123, 2001, 10639-49) of the sol-gel technique described above,that there was a strong inverse correlation between current and catalystparticle size.

Electrical Conductivity

Electrons generated on catalyst surfaces may be lost by interaction withadsorbed oxygen if they cannot readily be transferred through thecatalyst to the electronically conducting substrate. A further factor indefining cell current recovery therefore is the conductivity of thecatalyst. AC impedance spectroscopy measurements were used to determineconductivity characteristics of the catalyst and preliminary data arereported below and in FIG. 10. Sample 450rh100 450rh450 Thickness (mm)1.08 0.83 R (MΩ) 7.15 1.54 σ (S · cm⁻¹) 1.54 × 10⁻⁸ 1.9 × 10⁻⁸

It is envisaged that at the higher firing temperature, betterparticle-particle contact is achieved through partial sinterinig. Thisprovides better means for the transfer of electrons between grains andis consistent with the higher conductivity of the 450° C. sample.

Summary and Initial Interpretation

It is considered that the reduced tungsten (W(V)) is largely associatedwith surface hydroxylated tungsten ‘blue’ compositions of the generalform H_(x)WO_(3-x/2). The smaller the particle size, the higher thesurface area and a higher anticipated fraction of hydrated material.Potentially, this higher fraction could correlate with the number ofcharge carriers produced. The important (essential) function of thecathode is then to draw these carriers out of the catalyst and thistherefore relies on low electronic resistance of the catalyst.

INDUSTRIAL APPLICABILITY

The fuel cell described herein provides a photocatalytic reactor whichcan be employed throughout the range of applications envisaged forconventional direct oxidation fuel cells, and can also be applied tooxidation of hazardous organic pollutants in the anode reaction in thefuel cell of the invention. Thus the invention is applicable in thefield of water quality, environmental remediation technology, as areactor for vital fluids remediation and particularly to the disposal ofhazardous organic pollutants. Such a remediation reactor may be appliedin the oil and gas producing industry, e.g. in operational use withhydrocyclone separators and other fluid treatment operations, installedas an in-process facility to handle organic contaminants.

1. A photocatalytic reactor for generating an electric current byconsumption of a fuel containing organic material, said reactorcomprising a direct oxidation fuel cell including an anode and acathode, wherein the anode is a photocatalysis-assisted anode whichcomprises a photocatalyst on a surface of an electrically-conductivesubstrate so arranged as to be receptive to light, and alight-transmissive, proton-conducting membrane arranged between saidanode and the cathode, such that light passes through said membrane as afinal stage in an optical path to the photocatalyst, the photocatalystpromoting oxidation of the organic material and generating electron-holepairs, and said reactor is provided with means for introducing saidfuel, and means for connection to an external electrical circuit.
 2. Aphotocatalytic reactor according to claim 1, wherein saidproton-conducting membrane is formed of a light-conductive material. 3.A photocatalytic reactor according to claim 1, wherein the membranecomprises a proton-conducting glass.
 4. A photocatalytic reactoraccording to claim 1, wherein a proton-conducting metal film separatesthe proton-conducting membrane from the cathode.
 5. A photocatalyticreactor according to claim 1, wherein the membrane is a solidelectrolyte for conducting light onto the photocatalyst on the surfaceof the anode.
 6. A photocatalytic reactor according to claim 5, whereinthe light-conducting solid electrolyte is chemically modified to enhancetransmission of light onto the photocatalyst.
 7. A photocatalyticreactor according to claim 5, wherein the light-conducting solidelectrolyte is physically modified to enhance transmission of light ontothe photocatalyst.
 8. A photocatalytic reactor according to claim 1,wherein the anode comprises material exhibiting photocatalytic effects,said material comprising stable mixed valency metal oxide systems.
 9. Aphotocatalytic reactor according to claim 1, wherein the anode comprisesa material exhibiting photocatalytic effects selected from indium nickeltantalates, tungsten oxides, titanium oxides and combinations thereofwith at least one of tungsten oxides, molybdenum oxides and nitrogen.10. A photocatalytic reactor according to claim 1, wherein thephotocatalyst is activated by visible light (400-750 nm).
 11. Aphotocatalytic reactor according to claim 1, wherein the fuel is anaqueous liquid that contains organic pollutants which are capable ofbeing degraded by photocatalytic oxidation reaction at the anode.
 12. Aphotocatalytic reactor according to claim 1, wherein the cathode isselected from a mesh, a porous element or a perforated strip.
 13. Aphotocatalytic reactor according to claim 1, wherein the cathode is madeof a material selected from noble metals, catalytic alloys or ceramics.14. A photocatalytic reactor according to claim 1, operationallyconnected in a flow-through configuration to a fluid flow linecontaining a contaminated fluid containing organic contaminantsutilizable as fuel for the direct oxidation fuel cell.
 15. (canceled)16. An electrical power source comprising a plurality of modified directoxidation fuel cells, each of said cells having photocatalysis-assistedanode which comprises a photocatalyst on a surface of anelectrically-conductive substrate so arranged as to be receptive tolight, and a light-transmissive, proton-conductive membrane arrangedbetween said anode and a cathode, such that light passes through saidmembrane as a final stage in an optical path to the photocatalyst, thephotocatalyst promoting oxidation of organic material and generatingelectron-hole pairs, said cells being arranged to form a stack orbattery.
 17. A method of generating electrical power, particularly byconsumption of an organic fuel, by a photocatalytic reaction conductedin a direct oxidation fuel cell, said method comprising provision of afuel cell and a source of fuel for the cell, providing a photocatalyticsurface at an anode of the cell, wherein the anode is aphotocatalysis-assisted anode which comprises a photocatalyst on asurface of an electrically-conductive substrate so arranged as to bereceptive to light, and a light-transmissive, proton-conductive membranearranged between said anode and a cathode of the cell, such that lightpasses through said membrane as a final stage in an optical path to thephotocatalyst, the photocatalyst promoting oxidation of organic materialand generating electron-hole pairs, exposing the photocatalyst to light,and supplying fuel to the anode for photocatalytic oxidation, andgenerating electrical power as a result of said photocatalytic oxidationof the fuel.
 18. A method according to claim 17, wherein the fuel cellis contained in a photocatalytic reactor and the reactor is providedwith means for introducing said fuel and means for connection to anexternal electrical circuit.
 19. A method of disposing of an organicmaterial-polluted fluid comprising the application of said fluid in afuel supply to a fuel cell contained in a photocatalytic reactor asclaimed in claims
 1. 20. A method of preparing a photocatalyst for usein a photocatalytic reactor as claimed in claim 1, comprisingpurification of a salt containing an active catalytic component,recovering said purified component, and a heat treatment step applied tothe purified and recovered component in the range of from 100 to 450° C.or above.
 21. A method according to claim 20, wherein the catalyticcomponent comprises a tungsten oxide, and the heat treatment stepcomprises a drying step at about 100° C., followed by a subsequentconditioning step at about 450° C.
 22. A method according to claim 19,wherein recovering said purified component comprises deposition on acatalyst support and the catalyst support comprises a noble metal.